Components can be created directly from a computer model using such three dimensional printing processes which can be defined as processes that construct objects in layers using a computer model of the objects. Exemplary processes of this type are described, for example, in U.S. patent application of Sachs et al., Ser. No. 07/447,677, filed on Dec. 8, 1989, now U.S. Pat. No. 5,204,055, which application is incorporated by reference herein. As described in such application, a component can be produced by spreading powder (or other porous material) in a layer and then depositing a further material, e.g., a binder material, at specific regions of a layer as determined by a computer model of the component. The further material acts to bind the powder both within the layer and between layers. This process is repeated layer after layer until all layers needed to define the components have been printed.
The fundamental requirement that must be satisfied if three dimensional printing techniques are to be capable of manufacturing large quantities of product is to simultaneously satisfy the need to produce components at high rate and with high quality. Control of the quality of the part includes the control of dimensions and the control of its surface finish. Another aspect of component quality control is the control of the internal structure of the component so as to yield certain desired properties thereof, including a desired isotropic nature of the part, or a desired anisotropic nature of the part.
The rate at which parts can be created in three dimensional printing processes can be understood by recognizing that, typically, each cubic centimeter of binder that enters the powder bed binds approximately 1 cubic centimeter of powder and creates a portion of the part of approximately 2 cubic centimeters in volume. Thus, the overall volume of a part created is roughly twice the volume of binder deposited, the total volume varying depending on the type of powder used, so that it can be more or less than twice the volume of binder deposited. The attainment of a high production rate in a three dimensional printing process thus depends on depositing the binder at a high volume flow rate.
The binder can be applied using ink-jet printing techniques and the aforesaid Sachs et al. application describes a configuration of an apparatus where the device which delivers the binder is moved over the bed of porous material in a raster scan motion. Such configuration of a three dimensional printing machine, while not the only one possible, forms the principal basis for the invention described herein.
When the binder is delivered using ink-jet printing techniques, each droplet of binder enters the powder bed and joins together a number of powder particles to form a generally spherical "primitive" building element. FIG. 1 shows a micrograph of an exemplary spherical "primitive" element which is about 120 microns in diameter. The primitive shown therein was made by printing a single 80 micron diameter droplet of binder into a bed of powder. The binder used was colloidal silica and the powder was an alumina powder having an average particle size of 30 microns. As successive droplets are deposited, these spherical building elements overlap to form the finished component. The surface finish of the finished part is largely determined by the placement of the individual droplets and how they overlap. FIG. 2 shows a portion of a component where five droplets, shown in a simplified diagrammatic fashion, have been printed with some, relatively small, overlap. In FIG. 2, each of the circles represent diagrammatically a generally spherical building element of the type shown in FIG. 1 which results from an individual binder droplet. The surface of the printed part is defined by the surface 13, which can be seen to be somewhat rough due to the nature of the contours of the primitives. By contrast, FIG. 3, shows diagrammatically a portion of a component comprising a plurality of spherical elements which have been printed with a higher degree of overlap with the result that the surface 15 is relatively smoother than the surface 13. The problem of creating high quality parts at a high production rate can be seen to be a problem of how to deposit droplets at a relatively high rate while retaining the ability to determine and control the location of the droplet placement.
The requirements for a high production rate with high quality in a three dimensional printing process provide a significantly different challenge than the conventional printing of ink on paper. One significant difference is that in printing on paper it is the general practice to move the paper at high speeds past a stationary printhead. In three dimensional printing it is preferable, and often necessary, to move the printhead past a stationary bed of porous material because the bed of porous material is often relatively large and massive. Another difference is that, in conventional printing of ink on paper, the droplet separation needed to achieve high resolution printing is considered to be about 300 dots per inch (dpi). Thus, if the droplet position can be effectively controlled to occur at intervals of 1/300 inch, or approximately 85 microns, high resolution printing can be accomplished. However, such a 300 dpi droplet placement would be quite insufficient for the creation of three dimensional components. First, a 300 dpi placement would mean that the dimensions of the three dimensional object could only be controlled to within .+-.85 microns. Second, the surface finish of the object would be quite rough. For example, if the primitives of FIG. 2 represented primitives of 120 micron diameter and are placed 85 microns apart, the resulting surface would have a root-mean-square (RMS) roughness of approximately 6.0 microns and would result in a fairly rough surface. In fact, then the creation of components and surfaces in three dimensional printing processes is more complex than the situation represented by FIGS. 2 and 3 in that it is a 3-D problem, while conventional printing of ink on paper is a 2-D problem. Thus, in three dimensional printing, the placement of droplets must satisfy the requirement that the overlap between droplets must serve to bind together a self-supported component in three dimensions. In contrast, in conventional printing of ink on paper, only a 2-D image needs to be defined, and further, the paper itself provides the physical support for the image.